Project Summary: High levels of the oncoprotein c-Myc are found in a large percentage of human cancers. c-Myc is a transcription factor that has dual functions of promoting cell proliferation and programmed cell death, also known as apoptosis. Increased levels of c-Myc in the cell can disconnect these two functions, with increased proliferation and decreased apoptosis, leading to tumorigenesis. Recent work has elucidated that c-Myc stability and proliferative and apoptotic functions are regulated by two conserved phosphorylation sites, Serine 62 (S62) and Threonine 58 (T58). c-Myc is stabilized by phosphorylation of S62 and destabilized by subsequent phosphorylation of T58. The mutation of S62 to Alanine (S62A) in c-Myc, which removes T58 and S62 phosphorylyation, reduces induction of proliferation, while still inducing apoptosis. The mutation of T58 to Alanine (T58A) in c-Myc results in high levels of S62 phosphorylation and increased ability to promote proliferation coupled with decreased apoptotic capacity. The pattern of c-Myc phosphorylation of T58 and S62 in c-Myc can be altered in some human cancer cell lines, with increased S62 and decreased T58 phosphorylation. This suggests that unmutated c-Myc in cancer can be in a stabilized form with increased proliferative and decreased apoptotic function. The goals of the proposed research is to examine the role of T58 and S62 phosphorylation in regulating c-Myc's ability to promote proliferation or apoptosis in vivo, the underlying mechanism of this regulation, and how this contributes to c-Myc's oncogenic potential. The following specific aims will be pursued: 1) Characterize the effects of c-Myc T58 and S62 phosphorylation on its functions in vivo; 2) Determine the impact of T58 and S62 phosphorylation on c-Myc's oncogenic potential 3) Analyze the mechanisms by which T58 and S62 phosphorylation effect c- Myc activity. Tissue specific knock-in mice that express wild type, T58A or S62A c-Myc in thymocytes will be used to examine apoptotic and proliferative capacities of the c-Myc variants and their effects on T cell development and lymphomagenesis. The underlying molecular mechanisms T58 and S62 regulated c-Myc function will be examined in distinct T cell populations isolated from the mice. Relevance to public health: As a protein that is misregulated in a large percentage of human cancers, it is important to understand how c-Myc's dual functions to promote cell life and death can become disconnected leading to cancer for the development of future innovative therapuetics. [unreadable] [unreadable] [unreadable]